1. Field of the Invention
This invention relates to an intake system for a V-type engine, and particularly to an improvement in the arrangement of a collecting intake passage formed in an intake system for supercharging by the so-called dynamic supercharging effect (an inertia effect coupled with a resonance effect) of intake air.
2. Description of Prior Art
The technology of increasing the charging efficiency of intake air by supercharging intake air supplied to cylinders of an engine by utilizing an inertia effect and a resonance effect has been well known. In the case of supercharging by the inertia effect of intake air, a negative pressure wave of intake air is generated at an intake port at an initial stage of the air intake process for each cylinder as a corresponding intake valve is opened, and the intake air negative pressure wave is transmitted at a sonic speed toward the upstream side of an independent intake passage connected to an intake port, such that the negative pressure wave is reversed, namely, is converted into a positive pressure wave, at a designated volume part. Then this positive pressure wave is transmitted at a sonic speed toward the downstream side through the same passage such that it arrives at the intake port of the same cylinder at the end of a taking-in process or immediately before closing of the intake valve. This positive pressure wave causes intake air to be introduced into the combustion chamber of the cylinder in a manner which results in enhanced charging efficiency.
On the other hand, in the case where supercharging is effected by a resonance effect of intake air, plural cylinders of an engine are divided into plural groups. Each group comprises cylinders which are not sequentially timed in the ignition process and have independent intake passages which converge into one collecting intake passage (resonant intake passage). A pressure reversing part is provided at the designated position of this collecting intake passage. The pressure wave of intake air which is transmitted reciprocatively between the pressure reversing part and each cylinder is resonated in the collecting intake passage. This resonance generates a resonant pressure wave having an increased amplitude due to pressure vibrations generated individually at each cylinder. The resonant pressure wave causes intake air to be introduced into the combustion chamber of the cylinder in a manner which results in improved charging efficiency.
In a conventional V-type engine having a pair of opposite cylinder banks as shown by Laid Open Japanese Patent Application No. 56-52522, for example, cylinders of each bank are communicated with respective independent intake passages. An upstream end of this independent intake passage is connected to a pair of collecting intake passages, end portions of which are connected with each other to form a loop passage. This loop passage (two collecting intake passages) is arranged between the two cylinder banks.
As mentioned above, in such an intake system for a V-type engine a resonance state is generated between the individual cylinders of each bank. Supercharging is thus effected by the positive pressure wave of intake air generated by this resonance. That is by grouping cylinders which are not sequentially timed with respect to the air taking-in process for each bank, a pressure wave which is transmitted reciprocatively between a certain cylinder and a pressure reversing part (or joining part) of the collecting intake passage is resonated in the collecting intake passage to generate a resonance pressure wave. This resonance pressure wave results in supercharging because it causes the intake air to act on the air taking-in process at the other cylinder.
By increasing the length of each independent intake passage, supercharging can be provided both by the inertia effect of intake air during high speed operation of the engine and by the resonance effect.
However, in the case where a pair of collecting intake passages are arranged between two cylinder banks as mentioned above, a layout of long independent intake passage becomes complicated. Moreover, when the lengths of the independent intake passages are increased, the height of the collecting intake passage must also increase. Therefore, when this is applied to an engine for a passenger car, the engine compartment of the car increases in height.
As shown in U.S. Pat. No. 4,708,097, it is possible to arrange both of the collecting intake passages on one of the cylinder banks of the engine in such a fashion that one is mounted on the other and they share a common side wall. Such arrangement, however, makes it difficult to make the lengths equal for all of the independent intake passages corresponding to the cylinders of both banks. It is also difficult to make the independent intake passages of both banks substantially the same in shape (i.e. with respect to the degree of bending or radius of curvature) and there is a concern that the degree of bending of an independent intake passage communicating with a cylinder of one cylinder bank on the side where both collecting intake passages are arranged will be greater (i.e. the radius of curvature will be smaller) than that of an independent intake passage communicating with a cylinder of the other cylinder bank. Such differences in degrees of bending result in different intake resistances between the two cylinder banks, thereby causing irregular charging efficiencies for the intake air to the cylinders of the two cylinder banks.
In the above intake system intended to effect supercharging by the resonance effect of the intake air, the distance by which the pressure wave is transmitted is greater than the distance by which the pressure wave is transmitted when supercharging is effected by the inertia effect. Therefore, a good resonance effect can be obtained in the low/middle range of engine speeds in which the intake valve is opened for relatively long periods of time. This engine speed range is relatively limited and corresponds to the length of the intake passages. Accordingly, the resonance is not advantageous when supercharging will be needed over a wide range of engine speeds.
In order to overcome the above disadvantages, there has been suggested an intake system in which a resonance intake passage is composed of two kinds of passages, a first intake passage for high engine speeds with a relatively short intake passage (or channel) and a second intake passage for low engine speeds with a relatively long intake channel (as disclosed by Laid Open Japanese Patent Application No. 62-210219). In this manner, a good resonance effect can be obtained throughout a wide range of engine speeds by utilizing the first intake passage for high engine speeds and the second intake passage for low engine speeds.
However, since the above intake system is provided with two kinds of intake passages, i.e. intake passages for low and high engine speeds, the intake system is rather large in size and it becomes difficult to design a motor car with a compact engine compartment. In the V-type engine, the intake system of each cylinder bank is usually arranged above the cylinder bank so that the lengths of the independent intake passages to the cylinders of both banks can be made equal. This makes it difficult to arrange the intake passages so that they do not interfere with the engine compartment hood. This is especially true for transversely mounted V-type engines (i.e. in which the engine is disposed transversely with respect to the longitudinal direction of the vehicle), because having an intake system arranged above the forwardmost cylinder bank makes it difficult to design the engine compartment hood so that it is lower in the front than in the rear.
The present invention has been made with the above points in mind and has for its object, in the case where both collecting intake passages are mounted to each other and share a common wall to reduce and make substantially equal the degrees of bending of the independent intake passages of both banks so as to equalize the intake charging efficiencies of the cylinders of both banks and to generally improve the shape of each independent intake passage and its arrangement relative to the collecting intake passage.
Another object of the present invention is to make more compact the intake system of an engine provided with different intake passages for use at high and low engine speeds so as to facilitate vehicle design even for a vehicle with a low engine compartment hood.